1. Field of the Invention
This invention relates to lines used to connect subsea equipment to related equipment at or near the sea surface.
2. Description of the Related Art
Petroleum exploration and production is increasingly being conducted off-shore and at ever deeper locations. Typically, a mobile offshore drilling unit (“drilling rig”) is used to create a well. Once the well is completed, a production platform or a buoy is installed at the site to recover the petroleum products which may subsequently be loaded onto a tanker or pumped via pipelines to on-shore facilities.
Exploration and production platforms take many forms. The appearance and basic features of various types of offshore platforms are obvious to anybody skilled in offthore engineering and are widely described in technical literature. Examples include ships (mostly tanker-like Floating Production Systems—FPSs and FPSOs—FPSs with off-loading), semi-submersibles (including deep draft semisubmersibles), Tension Leg Platforms (TLPs), compliant and articulated columns and towers, guyed towers, SPAR platforms, jacket (fixed) platforms and jack-up rigs.
It is noted, that many riser, umbilical, hose, cable, etc. lines that are relevant to this specification have their top ends supported for example by buoys, columns, etc. that cannot be classified as platforms.
Lines that are relevant to this specification are used in order to:                transport fluids in both directions between locations at or near the surface and at or near the bottom (examples include import and export lines transporting hydrocarbons, water and gas injection lines, gas lift lines, etc.); in addition to transporting fluids, risers contain fluid pressure and support their structural loads,        transfer electrical and hydraulic power,        transfer information, including control, monitoring, data, telecommunication,        transfer loads (examples: tendons, tethers, cold tubing, etc., many risers deployed share mooring loads with ‘regular’ moorings).        
In particular, oil and gas petroleum products are recovered to the surface using production and test risers.
Export risers are used to transport petroleum products from fixed or floating structures, vessels or buoys to export pipelines that can be connected to other offshore structures or to locations onshore.
Water and gas injection risers as well as gas lift risers typically transport fluids from the surface to the vicinity of the seabed.
Drilling risers are used for drilling below the seabed. Drilling risers incorporate a drill-pipe that in addition to be used for actual drilling transports drilling mud from the surface inside its body. The mud and fragments of drilled rock, etc. are transported back to the surface inside the riser but outside the drill-pipe.
Workover risers are used for well maintenance, including transporting tools far below the seabed.
Umbilical lines perform many specific or combined roles, as required by technological needs.
Jumpers and hoses are used to transport fluids over limited distances, predominantly within limited range of water depth. Those can be used close to the surface, at or close to the seabed or anywhere within the water column.
Hybrid Risers incorporate combinations of some of lines listed above as examples and/or other types of lines that are bundled together. Other bundling configurations are also used offshore, which include piggy-backing, arranging lines in interconnected arrays (example: star-shaped array), stubbing hydraulic lines between connectors of drilling risers, etc.
Lines feature a variety of prior art configurations that are used in offshore and onshore engineering. The two major classes of lines include:                catenary lines (examples: flexible risers, Steel Catenary Risers—SCRs, umbilicals, hoses, jumpers, cables),        tensioned lines (examples: tensioned risers including freestanding and hybrid risers, and tendons or tethers).Most of the said lines are relevant to this specification and they are referred to herein as ‘lines’. Many line configurations are used in marine engineering, their basic features are well-known to those skilled in the art, and they are well described in technical literature.        
For example Barltrop1 depicts and describes a representative (but not complete) selection of prior art line configurations used in offshore engineering. Many of the line configurations known are referred to elsewhere in this specification. 1 Barltrop N. D. P., Floating Structures: a guide for design and analysis, Vol. 2, The Centre for Marine and Petroleum Technology, Publication 101/98, 1998.
U.S. Pat. No. 5,222,453 demonstrates a use of mass enhancing devices mounted on mooring lines and utilized to modify dynamic motions of a moored structure, without affecting static loads in the mooring system, where axial line dynamics is of primary importance. These were of little relevance to this invention that is related to different kinds of lines like risers, umbilicals and hoses.
For the purpose of this specification, in most cases, the details of line description (example: flexible riser, hose or umbilical or even an SCR) is of secondary importance or even of no importance. This is because different lines are subject to the same physics, the same harsh environment and there are many similarities between equipment used with various line configurations, with lines constructed in differing ways, (including using different materials) as well as lines used for vastly differing functional purposes.
A general description and explanation follows of technical issues in offshore and onshore engineering, including problems, as relevant to this invention, as well as that of prior art in the mitigation of some of the said problems.
In particular a simple (free-hanging) catenary configuration, as well as in many implementations of other line configurations are known to experience significant movement near the seabed and interactions with the seabed and/or with structures at the seabed ends of the lines.
It is known to anybody skilled in the art that the Touch Down Zone (TDZ) riser dynamics is of primary design importance in offshore line engineering. The design issues particularly relevant to line engineering are:                Dynamic bending, involving high dynamic bending stresses, particularly in the TDZ; any increase in the dynamic bending stresses reduces the fatigue life of the line structure wherever it occurs.        Dynamic axial loads that also reduce the fatigue life wherever they cause dynamic stresses.        Reductions in the dynamic tension, which includes dynamic line compression with a possibility of buckling or bird-caging of the line.        
The dynamics of the TDZ and that anywhere along the lines is originated at the top of the line. That is mostly due to dynamic motions of the supporting structure or vessel that are directly transferred to the line via the line hang-off. Those propagate along lines as dynamic transverse and axial deformations (waves) and as such are responsible for dynamic stressing of the lines.
Additionally, hydrodynamic forces due to waves, also induce in said lines transverse and axial deformations that are also propagated along said lines as transverse and axial waves (deformations).
The dynamic deformations travel along the lines towards the seabed and back towards the surface due to reflections that take place in the Touch-Down Zone, down along a connected pipeline (if applicable) beyond the TDZ, at the line hang-off and at intermediate locations within the water column. Those intermediate reflections occur wherever the line mass per unit length (including the added mass) changes along the length on said line. The physics of said reflections and wave propagation is well understood and it follows the same laws as does the physics of wave reflections in any media. The extent of these line movements, together with the variations in the values and the sign of the effective tension and the variations in the radii of curvature of the said lines, in particular but not exclusively near the seabed, are mitigated by this invention.
Risers and mooring lines are used in many design configurations that include various applications of negatively buoyant clump weights and distributed weights, approximately neutrally buoyant lines and devices as well as positively buoyant discrete and distributed, positively buoyant elements and segments. By that a line is neutrally buoyant it is meant herein that the line is either neutrally buoyant or, more often, approximately neutrally buoyant. Depending on the stage of their use and on the density of the surrounding seawater or fresh water, the fact whether or not a line is positively, neutrally buoyant or negatively buoyant also depends on the density or densities of materials used, materials contained, including fluids contained inside a line or lines. Many materials used degrade and absorb water while in service, accordingly, it is a common practice to supply any buoyant devices as well as any devices desired to be approximately neutrally buoyant with some excess of positive buoyancy.
Quasi static shapes of the lines can be approximated with the use of ideal catenary equations.
The approximation involved is due to neglecting any bending stiffness of the said line or the said line segment. Catenary equations typically approximate well shapes of mooring lines and flexible lines like hoses, flexible pipe, cables and umbilicals. In addition to these, entire SCR lines of the simple (free hanging) configurations as well as for example lazy wave SCRs are well approximated with catenary line equations in deep water, because in the said conditions bending stiffness of even a rigid metal line is negligible in comparison with the scale of the structure deployed. These include all configurations known of said flexible and said rigid lines used in offshore engineering, some of which are described by Barltrop1. 1 Barltrop N. D. P., Floating Structures: a guide for design and analysis, Vol. 2, The Centre for Marine and Petroleum Technology, Publication 101/98, 1998.
With regard to the In-Plane (IP) shapes of the catenaries, for lines with distributed weight and buoyancy, (as it follows from the catenary equations) it is noted, that:                negatively buoyant catenary segments have their curvature ‘bulging’ downwards,        neutrally buoyant or near vertical lines are well approximated with straight lines,        and positively buoyant segments have their curvature ‘bulging’ upwards.        
Discrete clump weights and buoyant connections (single clamps and buoys) IP result in local ‘sharp’ points or ‘spikes’ on catenaries, whereas:                Downward spikes occur at negatively buoyant devices;        No spikes are present at neutrally buoyant devices;        Upward spikes occur at positively buoyant devices.        
Three dimensional, real catenaries have their shapes also modified in the Out-of-Plane (OOP) direction due to drag in a current. The above observations for the said IP shapes can be generalized to the shape modifications OOP in the following ways:                Relative differences in drag between segments result in more or less pronounced bulging with a uniform current, for segments generating higher or lower drag, respectively;        Localized (discrete) drag devices that generate higher drag are associated with sharper spikes.        
Accordingly, in three dimensions, the combinations of the submerged weight (positive, neutral or negative) and drag forces are responsible for quasi-static shapes of catenary segments, while clump weights, tethered or clamped buoys are responsible for spikes in the shapes, because of the combinations of the weight, buoyancy and drag forces. Drag forces can significantly modify shapes of catenaries, depending on the local strength of current (i.e. current velocity) and the drag coefficient of any particular line segment or a device incorporated. Currents are seldom uniform along said lines. Typically both their velocities and directions vary along the line.
In addition to the above described, quasi-static effects of the weight, buoyancy, and current drag forces, which will be used to optimize the use of this invention on particular examples, line dynamics plays a significant part in the dynamic behavior of the said lines.
Dynamic effects on lines used in offshore engineering can be very complex. The said lines typically experience dynamic wave action that dynamically modifies the said line configurations. Typically, the wave forces act as time variable drag forces and as time variable inertia forces, approximately as described by the Morison Equation. These are modified by the interactions between waves and currents that are complex, but for practical engineering systems it is usually acceptable to approximate the interactions by superposing currents with waves kinematically. Amplitudes of wave forces decrease along lines with the water depth, which in deep water means the force decreases (approximately exponentially) to practically nil at deep water segments of the said lines. In addition to said wave forces, said lines are often subjected also to dynamic resonant excitations due to Vortex Induced Vibrations (VIVs) in currents and waves. In addition to dynamic bending of lines and to their fatigue loading, VIVs are also responsible, wherever they occur, for the increase in the quasi-static drag on the line.
It should also be stated, that many of the said lines are attached at their top ends to floating structures that also move on waves. The motions of the said structures add to the wave generated and other motions of the lines, and they are directly transferred to said lines at their top ends attached to said floating structure. All these motions are transmitted dynamically as line deformation waves along the line catenaries (straight line segments included) both up and down the catenaries with differing velocities, dependent on a nature of the wave motion generated on the line.
In particular, axial waves are transmitted along said lines very fast, approximately at the speed of sound in the materials used.
Catenary tension waves are also transmitted with similar velocities along the line and they result in movements of the entire catenary, almost like a rigid body. A significant portion of the heave transferred to said line can result in motions of this kind and the deformations travel along said lines slightly slower than the acoustic waves. Other motions, together with the remaining part of the heave motion tend to be transmitted along said lines much slower, as transverse deformation waves.
Static and dynamic coupling exists between the torsion of the line and its bending wherever three dimensional bending occurs (torsion waves tend to travel along said lines faster than transverse deformation waves). The latter interactions result in some redistribution of the corresponding oscillation energies, however the amplitudes resulting tend to be small in practice and in most cases these phenomena can be disregarded.
For said lines having multilayer structure, where different materials are used in different layers the wave transfer velocities tend to differ between layers, however the structurally dominant layers tend to control the motions.
All said waves traveling along said lines are subjected to reflections on the lines whenever the mass and line directions change, as well they are subject to dynamic interactions with the seabed. The quasi-static and momentary dynamic shapes of catenary lines are tension controlled, and it is the property of the catenaries, that the effective tension is the lowest at and near the touch down areas to the seabed (or at ends connected to subsea structures), where the (effective) tension-controlled line stiffness is the lowest.
It is often the case that the effective tension near the touch-down becomes periodically negative, making the line susceptible to local buckling, which usually is not desirable and sometimes it is completely unacceptable (example fiber-optic lines).
Riser and pipeline engineering codes that are also relevant to umbilical lines, cables, etc. recommend effective dealing with the problem of the occurrence of negative dynamic effective tensions. These decreases in the effective tension are often accompanied with dynamic reductions in the line radii of curvature. Bird-caging of umbilical or cable lines can occur, rigid or flexible pipes usually have some built-in resilience, but complex local increases in fatigue damage typically result. Often, in presently known designs it is difficult to increase the effective tension and to increase the minimum dynamic bending radii to acceptable levels. Increasing the horizontal tension in the catenaries, which increases also the quasi-static, average effective tension at the touch-down in many known designs is known to often make the dynamic effects described above even worse.
It is noted that the said effective tension is a physical value responsible for the line shape and buckling behavior for lines that include fluid contained pipes, as described by Young and Fowler2. Internal fluid pressures inside a rigid or flexible pipe, as well as pressures inside umbilical tubes, together with the external hydrostatic pressure in the surrounding water affect the actual (wall) tension in the line or lines, whereas said effective tension governs the behavior of the line. For some lines, like cables, electrical umbilicals or solid rods, effective tension and the actual tension are equal and they are simply known as tension. However, with the above understanding the term effective tension is used herein for all types of lines, whenever required, because it is more general. 2 Young R. D., Fowler J. R., Dynamic Analysis as an Aid to the Design of Marine Risers, Transactions of the ASME, Journal of Pressure Vessel Technology, Vol. 100, May 1978.
In particular, the said touch down zone line dynamics is in presently known designs both significant and troublesome for simple, free hanging catenary lines attached to floating structures. Examples of floating structures that are associated with the biggest motions are tankers (FPSs and FPSOs), particularly when they are bow or stern turret-moored. On such designs, all the risers, umbilicals, cables and mooring lines are attached to the turret. The motions of the FPSs and FPSOs are typically the biggest at their bows and sterns, which are also typical locations for turrets. However, many FPSs and FPSOs feature wide beams in order to maximize their deck areas, and accordingly line tops attached to riser banks on vessel sides can also experience high motions. Single Buoy Moorings (SBMs) and Semi-submersible vessels can also transfer considerable motions to catenary lines. Top-end induced motions are typically smaller for articulated or compliant towers, Tension Leg Platforms (TLPs), SPARS, including Truss SPARS and other deep draught vessels, but they are by no means negligible.
In the presently known designs the most effective way of mitigating the problem is to use one of the wave or ‘S’ configurations, as described by Barltrop1. 1 Barltrop N. D. P., Floating Structures: a guide for design and analysis, Vol. 2, The Centre for Marine and Petroleum Technology, Publication 101/98, 1998.
The wave or ‘S’ configurations are sometimes unavoidable in shallow water conditions and/or with strong variable currents. Because of large horizontal motions of the vessel in these situations (that can be caused by waves, by variable currents or both), one of these configurations has to be selected in order to reduce the maximum dynamic effective and wall tensions in the catenary to an acceptable level.
In ultra deepwater conditions, the selection of for example lazy wave for a flexible, cable or an umbilical line or for SCRs can also be the best solution because of the line weight in its operational or installation configuration. In particular, at present, it might be not possible to use larger diameter single pipe or Pipe-in-Pipe (PIP) SCRs on some fields, where smaller diameter freehanging configurations are at present used. This is because the selection of a simple (freehanging) catenary configuration would have resulted in very high hang-off loads. These would have become even higher in a case of an accidental flooding of the line with seawater that might inadvertently happen during installation or in operation. In such cases using a freehanging catenary might be impossible, because the excessive hang-off load resulting might be too high to handle. Similarly, there might be no installation vessel available anywhere in the world, to handle such a heavy pipe during its installation; or in particular to handle such a large diameter pipe or Pipe-In-Pipe, in a case of an accidental flooding with seawater. The feasible solutions in such cases would be to use wave or ‘S’ configurations, decrease loads with auxiliary buoyancy, or to use a larger number of smaller diameter lines that are lighter, so that the maximum tension loads can be handled.
To summarize lazy wave, steep wave, pliant wave, lazy and/or steep ‘S’ configurations according to prior art are used primarily because of two sets of reasons:                In shallow water in order to deal with large horizontal motions of their top supports in waves and/or currents;        In ultra deepwater in order to decrease the maximum (tensile) loads;        An added advantage is some reduction in touch-down or bottom end dynamics.It is noted, that the average effective tensions at the top of the lower negatively buoyant segments of lazy and steep wave and ‘S’ configurations may be of similar order of magnitude as those at the line hang-offs. It is also noted, that for the same reasons using modified wave or/and ‘S’ configurations featuring more than one buoyant segment (buoy) are known. In such cases the subdivisions of the negatively buoyant segments of the catenaries is in known designs in segments featuring comparable lengths and comparable maximum tension loads resulting from similar design philosophy as that used for the design of the single wave and/or ‘S’ configurations. This is because of the same reasons of maximizing the flexibility of the line (shallow water) or minimizing the maximum loads (ultra deepwater). However, it is noted that:        The use of the configurations in question, as implemented in prior art, results in the increase of the suspended lengths used (and in the corresponding increase in costs of the installation that adds to the cost of the associated ‘additional’ hardware used);        The selection of one of these configurations in prior art is because of one of the underlying reasons listed above; in the prior art these line configurations are, not selected because of the said added advantage. The reasons are economical, as specified directly above.        
Because of their higher costs, the energy industry tends to avoid using said wave or ‘S’ configurations in conditions where simple catenaries can be made feasible. However, even for lazy wave, lazy S or compliant wave configurations, where partial dynamic decoupling can occur, Barltrop1 states that touchdown line movements could also be significant. 1 Barltrop N. D. P., Floating Structures: a guide for design and analysis, Vol. 2, The Centre for Marine and Petroleum Technology, Publication 101/98, 1998.
Another known way of obtaining a partial reduction in the said line touchdown dynamics is a partial decoupling of motions by using a clump weight low on a catenary. This method tends to be only partially effective, because this makes the catenary above the clump weight steeper and it can result in the heave motions being transferred more easily down to the location of the clump weight. It also increases both the mass and the kinetic energy of the system moving, which would also tend to work in the opposite direction to that, which is desired. However, due to the enhanced dynamic decoupling effect in this solution together with careful tuning of the mass added and of its location to the particular dynamic wave spectra prevailing on a field, a partial improvement can be achieved.
Garret et al (Steel Lazy Wave on a Turret Moored FPSO, DOT 2002 Deep Offshore Technology Conference) depict a ‘traditional’ Lazy Wave SCR featuring a buoyant segment having a length of approximately 33% of the water depth.
Wu and Huang (The Comparison of Various SCR configurations for Bow Turret Moored FPSOs in West Africa, ISOPE 2007) present calculations for a Lazy Wave SCR utilizing a buoyant segment of approximately 18% of the water depth. They called their configuration Mini Lazy Wave.
Neither of the above two designs have ever been constructed. OTC 20180 co-authored by this inventor mentions Lazy Wave SCRs installed in 2009 offshore Brazil (BC-10 FPSO). The other co-author formally supervised mathematical modeling work demonstrated in OTC 20180 and provided editorial help.